In semiconductor manufacture, microlithography is used in the formation of integrated circuits on a semiconductor wafer. During a lithographic process, a form of radiant energy such as ultraviolet light, is passed through a mask or reticle and onto the semiconductor wafer. The mask contains opaque and transparent regions formed in a predetermined pattern. A grating pattern, for example, my be used to define parallel spaced conducting lines on a semiconductor wafer. The ultraviolet light exposes the mask pattern on a layer of resist formed on the wafer. The resist is then developed for removing either the exposed portions of resist for a positive resist or the unexposed portions of resist for a negative resist. The patterned resist can then be used during a subsequent semiconductor fabrication process such as ion implantation or etching.
As microcircuit densities have increased, the size of the features of semiconductor devices have decreased to the submicron level. These submicron features my include the width and spacing of metal conducting lines or the size of various geometric features of active semiconductor devices. The requirement of submicron features in semiconductor manufacture has necessitated the development of improved lithographic processes and systems. One such improved lithographic process is known as phase shift lithography.
With phase shift lithography the interference of light rays is used to overcome diffraction and improve the resolution and depth of optical images projected onto a target. In phase shift lithography, the phase of an exposure light at the object is controlled such that adjacent bright areas are formed preferably 180 degrees out of phase with one another. Dark regions are thus produced between the bright areas by destructive interference even when diffraction would otherwise cause these areas to be lit. This technique improves total resolution at the object and allows resolutions as fine as 0.25 .mu.m to occur.
In general, a phase shift mask is constructed with a repetitive pattern formed of three distinct layers of material. An opaque layer provides areas that allow no light transmission, a transparent layer provides areas which allow close to 100% of light to pass through and a phase shifter layer provides areas which allow close to 100% of light to pass through but phase shifted 180 degrees from the light passing through the transparent areas. The transparent areas and phase shifting areas are situated such that light rays diffracted through each area is canceled out in a darkened area therebetween. This creates the pattern of dark and bright areas which can be used to clearly delineate features of a pattern defined by the opaque layer on a photopatterned semiconductor wafer.
Recently, different techniques have been developed in the art for fabricating different types of phase shifting masks. One type of phase shifting mask, named after the pioneer researcher in the field, M. D. Levenson, is known in the art as a "Levenson" phase shift mask. Such a mask be formed on a transparent quartz substrate or the like. An opaque layer, formed of a material such as chromium, is deposited on the quartz substrate in a pattern which carries the desired pattern. Phase shifting areas on the mask may be formed by depositing a phase shifting material over the opaque layer or by forming areas of the quartz substrate with a decreased thickness.
A representative process for forming a Levenson phase shift mask is shown in FIGS. 1A-1D. In FIG. 1A, a transparent quartz substrate 10 has a film of an opaque material 12 such as chromium (CR) deposited thereon. The opaque material 12 ray be deposited on the quartz substrate 10 using a suitable deposition process such as sputtering, chemical vapor deposition (CVD) or electron beam deposition (EBD).
Next, and as shown in FIG. 1B, photoresist is deposited and developed to produce a patterned layer of resist 14. The opaque material 12 is then etched through the layer of resist 14 forming a repetitive pattern of openings 16 through the opaque material 12. The quartz substrate 10 under every other one of these openings 16 will eventually become the light transmission areas 22 on the finished mask. The quartz substrate 10, under the other half of the openings 16 in an alternating pattern, is etched to a reduced thickness to produce the phase shifting areas 20 on the finished mask 26 (FIG. 1E).
Following formation of the openings 16, the layer of resist 14 is stripped. Next, and as shown in FIG. 1C, a second layer of resist 18 is deposited over the opaque material 12. This second layer of resist is sometimes referred to as the "phase layer". The second layer of resist 18 must be realigned with the pattern of the initially formed openings 16, such that half of the openings 16' are exposed and half of the openings 16" are covered with resist. This defines the alternating pattern of opaque light blockers 24, light transmission areas 22, and phase shifting areas 20 on the quartz substrate 10 in the finished mask 26 (FIG. 1E).
Accordingly, and as shown in FIG. 1D, the quartz substrate 10 is etched through the exposed openings 16' to form the phase shifting areas 20. After stripping of the second layer of resist 18, and as shown in FIG. 1E, the finished mask 26 includes a repetitive pattern of opaque light blockers 24, phase shifting areas 20 and light transmission areas 22.
The phase shifting areas 20 of the quartz substrate 10 have a reduced thickness. When the finished mask is used in a lithography process, the light passing through a phase shifting area 20 is shifted in phase relative to light passing through an adjacent light transmission area 22, which must travel through the full thickness of the quartz substrate 10. In the finished mask each phase shifting area 20 preferably has a thickness that produces a 180.degree. (.pi.) phase shift for light passing therethrough relative to light passing through the light transmission areas 22. This process for forming a Levenson phase shift mask 26 is sometimes referred to as a subtractive process, because substrate material is removed to form the phase shifting areas 20.
A problem with this method of forming a phase shifting mask 26 is that it is difficult to realign the second layer of resist 18 (FIG. 1C) with the openings 16 (FIG. 1B) initially formed in the layer of opaque material 12. For that reason, and as shown in FIG. 1C, the pattern of openings 16' in the second layer of resist 18 must be oversized. This helps to compensate for registration tolerances. Typically, these registration tolerances for the second layer of resist 18 will be on the order of .+-.0.35 to 0.50 .mu.m.
With the oversized second layer of resist 18, the corner portions 28 (FIG. 1C) of the opaque material 12 are left exposed. When the openings 16' are subsequently etched to form the phase shifting areas 20 (FIG. 1E), the corner portions 28 may also be etched to form notched portions 30 (FIG. 1D) on the opaque material 12.
Etching of the phase shifting areas 20 is typically accomplished using a dry plasma etch process such as reactive ion etching (RIE). Such an etching process combines plasma and ion beams along with low pressures to physically and chemically remove the quartz material. Such an etch process, however, has been known to etch the exposed corner portions 28 of the layer of opaque material 12 and re-deposit this material in unwanted areas of the substrate 10. This material redeposition may be due to the low vapor pressure of the by-products and the physical sputtering of the opaque material 12. The redeposited material may thus cause unwanted optical effects in the finished mask 26.
In view of these and other problems, there is a need in the art for improved methods of making masks for phase shifting lithography. Accordingly, it is an object of the present invention to provide an improved method of making phase shifting masks for lithography. It is a further object of the present invention to provide an improved method of making phase shifting masks in which an opaque layer of the mask is protected during an etching step of the fabrication process to prevent redeposition of the opaque material. It is yet another object of the present invention to provide an improved method of making masks for phase shifting lithography that is efficient, inexpensive and adaptable to large scale semiconductor manufacture. It is a still further object of the present invention to provide an improved method of making phase shifting masks that is compatible with different mask making techniques, such as a "voting" technique, wherein phase shifting areas are etched a multiple of times.